Within the SwiftUI neighborhood, many individuals provide you with their very own model of a conditional view modifier. It lets you take a view, and solely apply a view modifier when the situation holds. It usually seems to be one thing like this:
extension View {
@ViewBuilder
func applyIf<M: View>(situation: Bool, rework: (Self) -> M) -> some View {
if situation {
rework(self)
} else {
self
}
}
}
There are numerous weblog posts on the market with comparable modifiers. I feel all these weblog posts ought to include an enormous warning signal. Why is the above code problematic? Let’s take a look at a pattern.
Within the following code, now we have a single state property myState. When it adjustments between true and false, we wish to conditionally apply a body:
struct ContentView: View {
@State var myState = false
var physique: some View {
VStack {
Toggle("Toggle", isOn: $myState.animation())
Rectangle()
.applyIf(situation: myState, rework: { $0.body(width: 100) })
}
}
}
Apparently, when working this code, the animation doesn’t look easy in any respect. For those who look intently, you may see that it fades between the “earlier than” and “after” state:
Here is the identical instance, however written with out applyIf:
struct ContentView: View {
@State var myState = false
var physique: some View {
VStack {
Toggle("Toggle", isOn: $myState.animation())
Rectangle()
.body(width: myState ? 100 : nil)
}
}
}
And with the code above, our animation works as anticipated:
Why is the applyIf model damaged? The reply teaches us rather a lot about how SwiftUI works. In UIKit, views are objects, and objects have inherent id. Because of this two objects are equal if they’re the identical object. UIKit depends on the id of an object to animate adjustments.
In SwiftUI, views are structs — worth varieties — which implies that they do not have id. For SwiftUI to animate adjustments, it wants to check the worth of the view earlier than the animation began and the worth of the view after the animation ends. SwiftUI then interpolates between the 2 values.
To grasp the distinction in conduct between the 2 examples, let us take a look at their varieties. Here is the kind of our Rectangle().applyIf(...):
_ConditionalContent<ModifiedContent<Rectangle, _FrameLayout>, Rectangle>
The outermost kind is a _ConditionalContent. That is an enum that can both include the worth from executing the if department, or the worth from executing the else department. When situation adjustments, SwiftUI can’t interpolate between the previous and the brand new worth, as they’ve differing types. In SwiftUI, when you have got an if/else with a altering situation, a transition occurs: the view from the one department is eliminated and the view for the opposite department is inserted. By default, the transition is a fade, and that is precisely what we’re seeing within the applyIf instance.
In distinction, that is the kind of Rectangle().body(...):
ModifiedContent<Rectangle, _FrameLayout>
Once we animate adjustments to the body properties, there are not any branches for SwiftUI to contemplate. It may simply interpolate between the previous and new worth and every part works as anticipated.
Within the Rectangle().body(...) instance, we made the view modifier conditional by offering a nil worth for the width. That is one thing that nearly each view modifier help. For instance, you may add a conditional foreground shade through the use of an elective shade, you may add conditional padding through the use of both 0 or a worth, and so forth.
Be aware that applyIf (or actually, if/else) additionally breaks your animations when you find yourself doing issues accurately on the “inside”.
Rectangle()
.body(width: myState ? 100 : nil)
.applyIf(situation) { $0.border(Coloration.purple) }
If you animate situation, the border is not going to animate, and neither will the body. As a result of SwiftUI considers the if/else branches separate views, a (fade) transition will occur as a substitute.
There may be one more downside past animations. If you use applyIf with a view that accommodates a @State property, all state shall be misplaced when the situation adjustments. The reminiscence of @State properties is managed by SwiftUI, primarily based on the place of the view within the view tree. For instance, think about the next view:
struct Stateful: View {
@State var enter: String = ""
var physique: some View {
TextField("My Subject", textual content: $enter)
}
}
struct Pattern: View {
var flag: Bool
var physique: some View {
Stateful().applyIf(situation: flag) {
$0.background(Coloration.purple)
}
}
}
Once we change flag, the applyIf department adjustments, and the Stateful() view has a brand new place (it moved to the opposite department of a _ConditionalContent). This causes the @State property to be reset to its preliminary worth (as a result of so far as SwiftUI is worried, a brand new view was added to the hierarchy), and the consumer’s textual content is misplaced. The identical downside additionally occurs with @StateObject.
The difficult half about all of that is that you just won’t see any of those points when constructing your view. Your views look advantageous, however possibly your animations are a bit of funky, otherwise you typically lose state. Particularly when the situation would not change all that always, you won’t even discover.
I might argue that the entire weblog posts that recommend a modifier like applyIf ought to have an enormous warning signal. The downsides of applyIf and its variants are under no circumstances apparent, and I’ve sadly seen a bunch of people that have simply copied this into their code bases and had been very proud of it (till it grew to become a supply of issues weeks later). In reality, I might argue that no code base ought to have this operate. It simply makes it manner too simple to by accident break animations or state.
For those who’re inquisitive about understanding how SwiftUI works, you may learn our ebook Pondering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend one in all our workshops.